Early active sonar systems relied on the audio sense of the human operators to make detection of targets. Sonar systems generated acoustical signals which were transmitted through the water environment of the oceans to reflect off of targets and return as echo signals. The returned signal was converted to an electrical signal, conditioned, and fed to the human operator where he could listen to the echo signal through the use of headphones. It was the operator's ability to resolve subtle differences in acoustical sounds that allowed him to make a detection of a target and to evaluate the operational conditions for various targets.
Recently, more emphasis has been put on the implementation of receiver/processor hardware to provide visual presentation to the human operator. This is to enhance so called multi-parameter estimation capability, e.g., range, Doppler, azimuth, etc. The human operator's auditory role thus diminished.
Further, the transmit frequency of active sonar systems has been lowered in order to achieve longer ranges within the ocean medium. Typical operational frequencies have been changed from 26-38 kHz for the older systems to 6.5-9.5 kHz for more modern systems. This is a decrease by a factor of four and has effected the resultant hearable frequency offset caused by Doppler shift which had been monitored aurally by the operator. The decrease in Doppler sensitivity is particularly noticeable for shallow-water and/or low-velocity target conditions for which reverberation can more easily mask target echoes. These are generally thought to be the primary reasons for the decreased use of the audio capability or channel for active target detection.